1. Field of the Invention
The present invention relates to an objective lens system for use within a microscope which is employable in the ultraviolet range, in particular, in the far ultraviolet range, in which light has a wavelength of shorter than 300 nm.
2. Description of the Prior Art
It is commonly known in the art that a microscope has a property that, assuming the numerical aperture (NA) of an objective lens system used within the microscope remains constant, the shorter a wavelength of light used therein, the better its resolution limit. Thus, it is possible to observe a sample in greater detail by shortening the wavelength of illumination light. In addition, illuminating a sample with ultraviolet light may often result in fluorescence of stronger intensity discharged from a sample than fluorescence obtained by illuminating with visible light. Against this background, a microscope employable in the ultraviolet range is preferred in the art, because one obtains more information by observing a sample through such a microscope. Thus, an objective lens system for use within a microscope must be employable in the ultraviolet and/or far ultraviolet range.
Among known conventional objective lens systems which are employable in the ultraviolet and/or far ultraviolet range is, for example, an objective lens system for use within a microscope described in "Hikari Gijyutsu Contact," Volume 25, Number 2, Page 137 (Feb. 1987). This objective lens system is illustrated in FIG. 6.
In FIG. 6, an objective lens system 70 includes a first lens 71 made of fluorite, a second lens group 72 and a third lens group 73 disposed in that order from an object side (left-hand side of the figure) to an image formation side (right-hand side of the figure). The second lens group 72 includes two convex lenses 72b and 72c both made of fluorite and a concave lens 72a made of quartz. The second lens group 72 is formed by holding the concave lens 72a between the convex lenses 72b and 72c and joining the same to each other. The third lens group 73 is formed, in a similar manner to the second lens group 72, by holding a concave lens 73a made of quartz between two convex lenses 73b and 73c both made of fluorite and joining the same to each other.
Since the lenses 71, 72a to 72c and 73a to 73c are made of either quartz or fluorite, the objective lens system 70 is capable of transmitting ultraviolet and/or far ultraviolet light, and hence, employable in the ultraviolet and/or far ultraviolet range.
In addition to this, chromatic aberration can be corrected in the objective lens system 70, since the second lens group 72 is composed of the concave lens 72a made of quartz and the convex lenses 72b and 72c made of fluorite while the third lens group 73 is composed of the concave lens 73a made of quartz and the convex lenses 73b and 73c made of fluorite.
The convex lens 72b, the concave lens 72a and the convex lens 72c of the second lens group 72 are brought into optical contact and joined to each other. Similarly, in the third lens group 73, the convex lens 73b, the concave lens 73a and the convex lens 73c are brought into optical contact thereby to be joined to each other. At the current level of the art adhesive that transmits far ultraviolet light is not available yet. Further, in order completely eliminate reflection at junction surfaces between lenses, there is currently no option other than cementing by optical contact. Thus, in the process of manufacturing the objective lens system 70, the junction surfaces must be finished with extremely high accuracy, which results in high cost.
The inventor of the present invention has already suggested an objective lens system for use within a microscope in which such a problem is solved. See Japanese Patent Laid-Open Gazette Nos. 1-319719 and 1-319720. These literature references will be hereinafter referred to as the "precedent applications." FIG. 7 shows an objective lens system for use within a microscope, namely, objective lens system 60, according to an embodiment of the precedent applications. The objective lens system 60 includes lenses 61 to 63, which are made of either quartz or fluorite. The lenses 61 to 63, i.e., the first to the third lenses, are displaced in that order from an object side (left-hand side in the figure) to an image formation side (right-hand side in the figure) with preselected air spaces therebetween. This enables the objective lens system 60 to be employable in both the ultraviolet and far ultraviolet range. The lenses 61 to 63, as mentioned, are separated from each other; that is, the objective lens system 60 includes no junction surfaces. Thus, because there is no direct optical contact, the objective lens system 60 is free from the problems relating to manufacture cost of the system shown in FIG. 6.
The objective lens system 60 cooperates with an image formation lens system (detailed structure thereof will be given later) in order to form an image of an object to be observed on the focal plane of the image formation lens system at a predetermined imaging magnification M. In this case, the imaging magnification M is a ratio of the focal length f.sub.2 of the image formation lens system to the focal length f.sub.1 of the objective lens system 60, and is given as: EQU M=-f.sub.2 /f.sub.1 ( 1)
In general, an objective lens system is replaced with another objective lens system to change the imaging magnification while an image formation lens system is fixed. Objective lens systems for replacement are necessary for this end, each of the lens systems having a focal length different from the focal length f.sub.1.
The equation (1) indicates that an objective lens system for replacement which has a focal length of (f.sub.1 /5) is necessary to observe an object at the imaging magnification of 5.multidot.M. If all that is required is to provide the other objective lens system for replacement such that its focal length becomes (f.sub.1 /5), the necessary objective lens system for replacement merely has to have a size of a proportionally reduced objective lens system 60.
However, if the objective lens system 60 is replaced with the required objective lens system for replacement in which the focal length thereof is 1/5 that of the objective lens system 60, the distance between the required objective lens system for replacement and the object to be observed would have to be 1/5 that of the objective lens system 60 as long as the pupil of the microscope objective lens is fixed. This is extremely time-consuming as well as labor-consuming in that the microscope must be brought into focus once again from the beginning after the replacement, and therefore, would enormously deteriorate operation of the microscope. On the other hand, if the position of the object is fixed, the position of the pupil would have to be moved. This should also be avoided in an illumination system for illuminating the object, since positional changes of the pupil exerts unfavorable effects upon illumination conditions. In addition, such replacement causes the pupil size to be 1/5 times, which in turn causes remarkable changes in to amount of illumination light on the object.
Thus, in the case of obtaining an image of the object under a microscope at 5.multidot.M power, an objective lens system for replacement must have:
(a) a focal length being 1/5 that of the objective lens system 60;
(b) parfocality; that is, the property of eliminating the necessity of bringing a microscope into focus once again after replacement; and
(c) a pupil which is roughly the same in size as that of the objective lens system 60.